Haemoglobin & Oxygen (DP IB Biology)

Foetal & Adult Haemoglobin

• Haemoglobin is the molecule responsible for binding oxygen in our blood

  • They are globular proteins found in abundance in red blood cells

  • Each haemoglobin molecule consists of four polypeptide subunits

  • At the centre of each subunit is an iron-containing haem group with which oxygen combines

    • Each haem group can bind to one oxygen molecule

    • That means that each molecule of haemoglobin can transport four oxygen molecules

• Oxygen is one of the gases found in air and each of these gases exerts a pressure

  • The pressure of each gas in a mixture of gases is called its partial pressure

  • The symbol for partial pressure is p, therefore the partial pressure of oxygen can be denoted as pO₂

• Due to the shape of the haemoglobin molecule it is difficult for the first oxygen molecule to bind to its haem group

• However, after the first oxygen molecule binds, the haemoglobin protein changes shape, or conformation, making it easier for the next oxygen molecules to bind

  • This is known as cooperative binding

• The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen

  • In areas where there are high partial pressures of oxygen (such as the alveoli of the lungs), the affinity of haemoglobin for oxygen is high

    • This means haemoglobin and oxygen will bind easily

  • In areas where there are low partial pressures of oxygen (such as respiring muscle cells), the affinity of haemoglobin for oxygen is low

    • This means haemoglobin and oxygen will dissociate easily from each other

  • This ensures that haemoglobin can easily bind to oxygen in the lung capillaries to transport it to and then release it near respiring cells that require oxygen 

Foetal haemoglobin

• The haemoglobin of a developing foetus has a higher affinity for oxygen than adult haemoglobin

• This is vital as it allows a foetus to obtain oxygen from its mother's blood at the placenta

  • Foetal haemoglobin can bind to oxygen at low pO₂

  • At this low pO₂ the mother's haemoglobin is dissociating with oxygen

• We can represent the percentage saturation of haemoglobin at different partial pressures of oxygen as a graph

  • This is called the oxygen dissociation curve

• On a dissociation curve, the curve for foetal haemoglobin shifts to the left of that for adult haemoglobin

  • This means that at any given partial pressure of oxygen, foetal haemoglobin has a higher percentage saturation than adult haemoglobin

• After birth, a baby begins to produce adult haemoglobin which gradually replaces foetal haemoglobin
  

  • This is important for the easy release of oxygen in the respiring tissues of a more metabolically active individual

Foetal haemoglobin has a higher affinity for oxygen; its oxygen dissociation curve therefore lies further to the left than the curve of adult haemoglobin

• Haemoglobin has the ability to change shape, or conformation, once oxygen binds to it due to cooperative binding

  • Proteins like this are known as allosteric proteins as they can exist in multiple conformations

• Carbon dioxide is an allosteric inhibitor of haemoglobin

  • This means that when it binds to haemoglobin, it is more difficult for oxygen to bind to haemoglobin as the protein cannot change its conformation

  • This lowers the affinity of haemoglobin for oxygen

• Carbon dioxide has less of an allosteric effect on foetal haemoglobin

  • This enables foetal haemoglobin to have a higher affinity for oxygen even if carbon dioxide is bound to it